1. Field of the Invention
The present invention relates to methods for determining operating parameters of a motor vehicle and, more particularly, to the estimation of the steering angle position and body slip angle of a vehicle based upon data received from various vehicle sensors.
2. Description of the Related Art
Many motor vehicles today have electronic stability program (xe2x80x9cESPxe2x80x9d) systems which include an electronic controller which utilize data signals obtained from various vehicle sensors to make complex calculations and automatically implement actions based upon those calculations.
One of the values that is generally required by such ESP systems is the vehicle steering angle position which may be measured by a steering angle sensor (xe2x80x9cSASxe2x80x9d). There are different classes of SAS sensors. Some SAS sensors, such as Class III sensors, provide accurate values immediately after the ignition key is placed in the ON condition. Other SAS sensors, such as Class I sensors, provide values which are relative to the position of the steering wheel when the ignition key is turned to the ON condition. In other words, these sensors transmit a zero value signal when the key ON condition is initiated regardless of the actual steering angle position and processing of the SAS signal is required to compute an accurate steering angle value. A variety of different methods are known for processing a relative position SAS signal to estimate the actual steering angle.
The slip angle of the vehicle body, i.e., the angle between the vehicle travel direction and the vehicle heading, is another operating parameter of the vehicle which has an impact on the stability of the motor vehicle and may be used by a stability control system.
The present invention provides improved methods and systems for estimating the body slip angle and actual steering angle position of a motor vehicle based upon signals received from various vehicle sensors.
Vehicle dynamics models oftentimes include a variable representing the steering angle value of the vehicle. In vehicles having a relative position steering angle sensor the absolute or true steering angle, xcex4, is equal to the sum of the relative steering angle position, xcex4Uncenter, provided by the steering angle sensor, and the steering angle offset, xcex4off, this relationship is represented by the following equation: xcex4=xcex4Uncenter+xcex4off.
The inventors of the present application have developed a method of directly estimating the steering angle offset value which recognizes that, because the steering angle offset value is a constant value during any one discrete vehicle trip, the sum of the relative value of the steering angle and the offset may be substituted for the absolute steering angle value in a vehicle dynamics model. The inventors of the present invention have also utilized such substitutions to develop vehicle models which provide a mathematical expression in which the steering angle offset value, xcex4off, is a state variable and can be directly estimated from the mathematical expression. As used herein, a variable is a state variable with respect to a particular mathematical expression when it can be directly calculated from such mathematical expression.
After reformulating the vehicle model so that the steering angle offset value, xcex4off, is a state variable, the Kalman filter or a state observer may be applied to the reformulated vehicle model to facilitate the estimation of the steering angle offset value, xcex4off. The mathematical expression provided by the application of the Kalman filter or state observer to the reformulated vehicle model may also be used to provide for the estimation of other vehicle operating parameters such as the vehicle body slip angle and yaw rate. The measured vehicle operating parameters which form the input values for such mathematical expressions may include the relative steering angle position, yaw rate, longitudinal speed, and lateral acceleration of the vehicle.
The inventors of the present application have also developed a method for filtering the resulting values of the steering angle offset which uses a variable filter coefficient and which may also be used with alternative methods of estimating steering angle offset values. The filter coefficient is dependent upon an uncertainty factor which is defined so that it will vary with the dynamic behavior of the vehicle. The vehicle operating parameters which may be used in the determination of the uncertainty factor include the yaw rate, steering angle position and the lateral acceleration of the vehicle.
The invention comprises, in one form thereof, a system and/or method for estimating a steering angle offset value in a motor vehicle having a relative position steering angle sensor. The method includes providing a mathematical expression for calculating an estimated steering angle offset value, {circumflex over (xcex4)}off, based upon at least one measured value of a vehicle operating parameter. The mathematical expression is definable by selecting a mathematical model to describe the dynamic behavior of the vehicle which includes a first variable, xcex4, representing the steering angle of the vehicle; and substituting, for the first variable, xcex4, the sum of a second variable, xcex4Uncenter, representing the relative steering angle position and a third variable, xcex4off, representing the steering angle offset in the model to thereby provide the mathematical expression for calculating the estimated steering angle offset value, {circumflex over (xcex4)}off. The method also includes obtaining the at least one measured value for the vehicle and estimating the steering angle offset value using the at least one measured value and the mathematical expression.
The at least one measured value of the method may include the yaw rate of the vehicle, the relative steering angle position and the longitudinal velocity of the vehicle. The lateral acceleration of the vehicle may also be included in the measured values used with this method.
In alternative embodiments of this invention, the slip angle of the vehicle body may also be estimated from the mathematical expression provided by this method.
In one embodiment, the model used by this method may be equation (1.1) presented below and the substitution of the sum of the second and third variables into the model may result in equation (1.3) which is also presented below. The mathematical expression provided by the method may comprise either equation (1.4) or (1.6) which are presented below.
The method may also include a filtering process which uses a variable filter coefficient. The variable filter coefficient may be calculated using an uncertainty factor which is determined using at least one value from the group including the yaw rate of the vehicle, the relative steering angle position and the lateral acceleration of the vehicle.
The invention comprises, in another form thereof, a system and method for filtering a plurality of time indexed values in a process for determining a steering angle position of a vehicle having a relative steering angle sensor. The method includes estimating a plurality of time indexed steering angle offset values. The method also includes filtering a plurality of time indexed values which are a function of said plurality of time indexed steering angle offset values using a variable filter coefficient, Fk. The variable filter coefficient, Fk, is determined using an uncertainty factor, Uk. The uncertainty factor, Uk, is determined using at least one value from the group including the yaw rate of the vehicle, the relative steering angle position and the lateral acceleration of the vehicle.
For some embodiments of this method, the plurality of time indexed values are identical to the plurality of time indexed steering angle offset values. The uncertainty factor, Uk, may be determined using equation (2.5) presented below and the filtering of values may use equation (2.1) presented below.
The invention comprises, in still another form thereof, a system and method of controlling a vehicle. The method includes providing a mathematical expression for estimating first and second vehicle operating parameters. The mathematical expression is definable by selecting a mathematical model to describe the dynamic behavior of the vehicle and reformulating the model. The mathematical expression includes a model difference term which is dependent upon a difference between an estimated value of the second vehicle operating parameter and a measured value of the second vehicle operating parameter. A signal representing a measured value of the second vehicle operating parameter is obtained. The method also includes estimating the first vehicle operating parameter using the mathematical expression wherein calculation of an estimated value for the first parameter includes using the measured value represented by the signal in the model difference term. The estimated value of the first parameter is then output for use in the vehicle""s electronic control system.
The method may include first and second parameters which are selected from the group including the steering angle offset, the yaw rate and the body slip angle of the vehicle. In one embodiment of the invention, the first parameter is the steering angle offset and the second parameter is the yaw rate of the vehicle. The mathematical expression used to estimate the first parameter may be equation (1.6a) which is presented below.
The invention comprises, in another form thereof, a system and/or method for estimating a vehicle body slip angle which includes the use of vehicle dynamics model which is valid under operating conditions wherein the yaw rate or slip angle are changing to estimate the vehicle body slip angle. Different sets of input signals may be used with the model to perform the estimations. One such set of input signals, include signals representing the relative steering angle position, the longitudinal speed of the vehicle and the yaw rate of the vehicle. The lateral acceleration of the vehicle may also be included in the input signals. It is also possible for an input signal representing the absolute or actual steering angle position to be substituted for the signal representing the relative position steering angle position.
The present invention may be used with vehicles which include either an absolute position steering angle sensor (which provides a signal indicating the accurate steering angle position immediately upon starting the vehicle) or a relative position steering angle sensor (which requires the signal to be corrected to account for the steering angle offset).
For vehicles which have an absolute position steering angle sensor, embodiments of the present invention may be used to estimate the vehicle body slip angle. For vehicles which have a relative position steering angle sensor, alternative embodiments of the present invention may be used to estimate the steering angle offset value and/or the vehicle body slip angle. The present invention also provides a method for filtering the steering angle offset value.
An advantage of the present invention is that it provides a method for directly estimating both the body slip angle and the steering angle offset value of the vehicle.
Another advantage of the present invention is that it provides a relatively accurate value for the body slip angle and the steering angle position when the vehicle is in either a steady state or a non-steady state of operation. As used herein, a non-steady state of operation refers to a state of operation wherein the yaw rate or the body slip angle of the vehicle is changing.
Yet another advantage of the present invention is that it provides a method for estimating the body slip angle and steering angle which can utilize different sets of vehicle operating parameters as input data. For example, the method can be used to estimate the body slip angle and steering angle offset based upon the yaw rate, relative steering angle position and longitudinal velocity of the vehicle, or, the inputs may additionally include the lateral acceleration of the vehicle.
Still another advantage of the present invention is that it provides a filtering method for use in determining the steering angle offset which includes a variable filter coefficient wherein the filter coefficient varies in response to vehicle operating parameters which reflect the dynamic behavior of the vehicle such as the yaw rate, relative steering angle position and lateral acceleration of the vehicle.